Just South-West of town is the bus-station of Nan. It is still an old fashioned chaotic bus-station. I love it, it really gives you the feeling of traveling.
From here you can take the old "orange bus" to Thung Chang (passing Pua). And there are "Song Theauws" to bring you to town and other nearby villages.
From this bus-station you can take the bus to Chiang Rai, Chiang Mai or Bangkok. And of course to all places the bus-lines are passing like Phrae, Lampang, Phayao and Chiang Kham.
There are government busses but also the better & safer busses from "Nakhon Chai Air" and the "Green Bus". Near the bus-station are also some simple restaurants.